flyrotary@lancaironline.net Mailing List Archive

Charlie, an exit duct is a good idea especially where there are two separate cooling systems, 1 for water and 1 for oil. The exit duct on 1 system will isolate the exit air or cowling pressure from the other. Also if one cooler has some extra capacity the exit duct is handy for controlling air flow and temperature of that loop. I intend to have one on my next radiator install for both of those reasons; my radiator hangs under the engine from the mount and I have 2 oil coolers, 1 in each cheek.

I can’t tell from your photo how much room there is between the oil cooler and the heat shield or start of your exit duct but there should be close to 1 foot of unrestricted air flow from the center of your oil cooler. That side of the engine is busy with intake and exhaust so there’s not a lot of room to play with. The exhaust system and under cowl temperatures will benefit if the cooler on that side simply spills its exit air into the engine compartment and flows out over the exhaust. Lots to consider … but doing it your own way is always the most fun.

Jeff

From:	Charlie England <ceengland7@gmail.com>
Subject:	Re: [FlyRotary] Re: oil cooler mount question
Date:	Mon, 27 May 2013 14:01:10 -0500
To:	Rotary motors in aircraft <flyrotary@lancaironline.net>

Hi Jeff,

You may be right; in fact, you're probably right.

But...

Keeping radiant energy away from the heat exchanger was really a minor factor in my thought process. A data point: Every really fast (on reasonable HP) plane I've looked at (both liquid & air cooled) has used some form of exit duct to channel the heated air back into the free stream. The stuff in the pic is the beginnings of an exit duct for the oil cooler. It will be far from an optimum shape because of space limitations; maybe so far from optimum that I'll cut it away shortly after 1st engine start. But I'm going to take a shot at it to see what happens. 'Education & Entertainment', is why we get to do this, right?

I do have at least a layman's concept of why it might be a good idea to use an exit duct. Everyone struggles with pressure differential between front & back of heat exchangers. Everyone pays attention to the inlet side to keep the air flowing smoothly & to expand the area of the inlet so it slows down & gains pressure. So what happens on the exit side of the core? It expands again, into a massive area, where it slows down again, gaining more pressure but now with totally chaotic flow. This begs the question: What happens to the desired pressure differential across the core?

If there's an Aero guy who can tell me that the *idea* of exit ducts is bad, I'm willing to be educated, and to save some time, money & trouble. But the empirical evidence of their goodness is compelling, even with the torturous paths I've seen in Lyc installations.

So I'd like to give exit ducts a shot. It's easier to do the ducts now than after everything is built (I'll never go back once it's finished), & if it doesn't work, 10 minutes on a band saw & it will all be gone. :-)

Charlie

On 05/27/2013 07:18 AM, Jeff Whaley wrote:

Charlie, I used strips of 1/8” rubber the same way Ed described.

I had to rotate your photo 90 degrees to see the whole picture … it looks like you’re trying to heat-shield your intake with a plate behind the cooler; I think you’ll be more disappointed with the cooler’s performance than you will the intake’s performance if you leave that shield in place. Tightly diffuse the inlet side – leave the outlet side open – my 2 cents worth.

Jeff

From:	Charlie England <ceengland7@gmail.com>
Subject:	oil cooler mount question
Date:	Sun, 26 May 2013 12:13:09 -0500
To:	Rotary motors in aircraft <flyrotary@lancaironline.net>

I've started design/fabrication/redesign of my oil cooler mount/diffuser. Just as I had a 'final' answer on the mount & was contemplating the diffuser, it occurred to me that I didn't have any shock mount provisions for the heat exchanger. For those who have been flying for a while, have any of you hard mounted the cooler to its support structure? As the photo shows, the primary mount is direct to the reduction drive plate. I would simply add the shock mounts, but if I use this particular cooler (a CX Racing 30 row unit like the one on Tracy's 20B), I won't have any room. I do have the option of buying a 25 row or using the 19 row I already have on hand, but I'd rather have a bit too much capacity & throttle the exit air than start with too little & have to rebuild everything.

Thanks,

Charlie

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